Fibroproliferative disorders are characterized by the abnormal accumulation of fibrous tissue (“fibrosis”) that can occur as a part of the wound-healing process in damaged tissue. Such tissue damage may result from physical injury, inflammation, infection, exposure to toxins, and other causes. The fibroproliferative condition includes both a cell growth component and an extensive phase characterized by extracellular matrix accumulation. Examples of fibroproliferative disorders include dermal scar formation, keloids, liver fibrosis, lung fibrosis (e.g., silicosis, asbestosis), kidney fibrosis (including diabetic nephropathy), and glomerulosclerosis.
A variety of renal diseases can be classified as fibroproliferative. Glomerular (usually mesangial) cell proliferation occurs in many types of glomerulonephritides in conjunction with increased extracellular matrix accumulation (Iida et al., Proc. Natl. Acad. Sci. USA 88:6560–6564, 1991). For example, mesangial cell proliferation precedes glomerulosclerosis in the remnant kidney model (Floege et al., Kidney International 41:297–309, 1992), and experimental overexpression of growth factors such as PDGF-B and TGF-beta in the kidney induces cell proliferation, matrix accumulation, and glomerulosclerosis (Isaka et al., J. Clin. Invest. 92:2597–2601, 1993; Cybulsky, Curr. Opin. Nephropathy and Hypert. 9:217–223, 2000).
A number of vascular pathologies result from a combination of mesenchymal cell proliferation (smooth muscle and fibroblast-like) and extensive accumulation of extracellular matrix components. Such artery wall diseases as arteriosclerotic lesions, arteritis of various origins, and the vascular re-stenotic lesions that frequently follow angioplasty (Riessen et al., Am. Heart J. 135:357–364, 1998; Plenz et al., Arterioscler. Thromb. Vasc. Biol. 17:2489–2499, 1997; McCaffrey, Cytokine Growth Factor Rev. 11:103–114, 2000) are considered fibroproliferative. Other fibroproliferative responses include the fiborproliferative responses that occur in organs following transplant (e.g., heart transplants), at sites of vascular anastamosis, and at areas around catheter placements (e.g., arterio-venous shunts used for dialysis).
Bone formation, both physiologic and pathologic, can be described as the interplay between bone formation that results from proliferation of osteoblasts and production by them of extracellular matrix, and the replication of osteoclasts and their modulation of this matrix. Diseases where there is aberrant and ectopic bone formation, such as that occurring with prostate tumor metastases to the axial skeleton, are commonly characterized by active proliferation of the major cell types participating in bone formation as well as by elaboration by them of a complex bone matrix. These diseases can therefore be viewed as fibroproliferative.
Pulmonary fibrosis is a major cause of morbidity and mortality. Pulmonary fibrosis is associated with the use of high-dose antineoplastic agents (e.g., bleomycin) in chemotherapy and with bone marrow transplantation for cancer treatment. The development of lung disease is the major dose-limiting side effect of bleomycin. See, Tran et al., J. Clin. Invest. 99:608–617, 1997. Idiopathic pulmonary fibrosis (IPF) is another lung fibrotic disease characterized by a fibroproliferative response. Various factors, including aspiration and exposure to environmental pollutants may result in IPF (Egan, The Lancet 354:1839–1840, 1999). The standard treatment for IPF is oral glucocorticoids. However, lung function improves in less than 30 percent of patients who receive this treatment, and, regardless of treatment, the median survival is four to five years after the onset of symptoms. The proliferation of fibroblasts and the accumulation of interstitial collagens are the hallmarks of progressive organ fibrosis, however the biochemical mechanism of induction of lung fibrosis remains unclear (Ziesche et al., New Eng. J. Med. 341:1264–1269, 1999; Kuwano et al., J. Clin Invest. 104:13–19, 1999). Pulmonary hypertension results from a variety of initiating stimuli. Its progression is associated with pulmonary vascular sclerosis, which includes abnormal endothelial morphology and function, muscularization of normally nonmuscular peripheral arteries related to differentiation of pericytes, and medial hypertrophy and neointimal formation in muscular arteries as a consequence of hypertrophy, proliferation, and migration of resident smooth muscle cells and increased production of extracellular matrix components. These components include collagen, elastin, fibronectin, and tenascin-C. This fibroproliferative response can progress to life-threatening pulmonary arterial obstructive disease (Cowan et al., J. Clin. Invest. 105:21–34, 2000).
Liver (hepatic) fibrosis occurs as a part of the wound-healing response to chronic liver injury. Fibrosis occurs as a complication of haemochromatosis, Wilson's disease, alcoholism, schistosomiasis, viral hepatitis, bile duct obstruction, toxin exposure, and matabolic disorders. This formation of scar tissue is believed to represent an attempt by the body to encapsulate the injured tissue. Liver fibrosis is characterized by the accumulation of extracellular matrix that can be distinguished qualitatively from that in normal liver. Left unchecked, hepatic fibrosis progresses to cirrhosis (defined by the presence of encapsulated nodules), liver failure, and death.
In recent years there have been significant advances in the understanding of the cellular and biochemical mechanisms underlying liver fibrosis (reviewed by Li and Friedman, J. Gastroenterol. Hepatol. 14:618–633, 1999). Stellate (Ito) cells are believed to be a major source of extracellular matrix in the liver. Stellate cells respond to a variety of cytokines present in the liver, some of which they also produce (Friedman, Seminars in Liver Disease 19:129–140, 1999).
As summarized by Li and Friedman (ibid.), actual and proposed therapeutic strategies for liver fibrosis include removal of the underlying cause (e.g., toxin or infectious agent), suppression of inflammation (using, e.g., corticosteroids, IL-1 receptor antagonists, or other agents), down-regulation of stellate cell activation (using, e.g., gamma interferon or antioxidants), promotion of matrix degradation, or promotion of stellate cell apoptosis. Despite recent progress, many of these strategies are still in the experimental stage, and existing therapies are aimed at suppressing inflammation rather than addressing the underlying biochemical processes. Thus, there remains a need in the art for materials and methods for treating fibroproliferative disorders, including liver fibrosis.